In terms of future jobs, why does management of athletes and actors seems like a steady and ever growing business?

The specifics of this will vary to some extent depending on what country you live in. However, the general idea is that sports and entertainment are booming businesses in many countries. In addition, the people in those industries are making more and more money without really having the knowledge needed to negotiate contracts and manage the money they receive.

In many countries, sports and entertainment are growing as industries. We can see this in such places as India where the Indian Premier League (cricket) has become a major enterprise in the last few years. The people who make money through sports or entertainment are also typically not able to negotiate the hugely complicated contracts that they sign. Nor are they typically able to manage the money they make. For these reasons, the business of management of athletes and actors seems as if it will be a steady and increasing business.